1. Field of the Invention
The present invention relates to a process for hydroformylation of olefins.
More particularly, the present invention relates to a process for hydroformylation of olefins using a Group VIII noble metal-triarylphosphine complex as a catalyst which comprises removing organic high boiling point by-products accumulated in the catalyst liquid to maintain the amount of the catalyst liquid within a definite range.
2. Description of the Prior Art
It is a well known fact that Group VIII noble metal-triarylphosphine complex catalysts, particularly rhodium-triarylphosphine complex catalysts, and most especially a rhodium-triphenylphosphine complex catalyst, can be used advantageously as catalysts for the so-called hydroformylation reaction for producing aldehydes having one more carbon atom than the olefin starting material. In recent years, studies concerning these catalysts have been vigorously carried out, because they have excellent catalytic activity and produce straight chain aldehydes in a high yield. These Group VIII noble metal-triarylphosphine complex catalysts, which preferably contain free triarylphosphine, possess the advantage that after the aldehyde produced is separated from the resulting hydroformylation reaction liquid containing the complex catalyst by distillation, stripping with a blowing gas, etc., the residual solution containing the complex catalyst can be recirculated to the hydroformylation step, due to the high thermal stability thereof.
However, reaction by-products having a higher boiling point than the aldehyde produced (referred to herein as "organic high boiling point by-products"), which cannot be separated at the time of the separation of the produced aldehyde by distillation, etc., accumulate in the catalyst liquid when the catalyst liquid is reused with repeated circulation. Generally, the hydroformylation reaction of olefins is carried out industrially while continuously feeding definite amounts of reaction raw materials to a reactor having a certain volume. When these organic high boiling point by-products accumulate in the circulating catalyst liquid, it becomes impossible to maintain the operation in the reactor having a prescribed volume because of increasing of the volume of the catalyst liquid in an amount corresponding to accumulated volume. Even if the amount of the organic high boiling point by-products produced by an individual reaction cycle is very small, the accumulated amount becomes remarkably large due to repetition of the cycle of (1) reaction, (2) separation of the aldehyde produced, and (3) recirculation. Accordingly, it is necessary that not only the aldehyde produced, but also the organic high boiling point by-products, be separated from the reaction system by some means in an amount corresponding to the amount produced.
The following four processes are known for removing the organic high boiling point by-products in order to maintain the amount of the catalyst liquid within a definite range:
(1) a process which comprises purging the catalyst liquid containing said organic high boiling point by-products in a definite amount, corresponding to the amount of organic high-boiling point by-products formed; PA1 (2) a process which comprises introducing a large amount of circulating gas into the catalyst liquid containing the organic high boiling point by-products to carry out gas stripping; PA1 (3) a process which comprises selectively removing the organic high boiling point by-products by an extracting treatment or an adsorption treatment, etc.; and PA1 (4) a process which comprises distilling the catalyst liquid containing the organic high boiling point by-products to selectively distill off the organic high boiling point by-products alone.
However, these processes have some problems in the industrial application thereof.
First, according to the process (1), even though the by-products produced can be removed, the Group VIII noble metals and triarylphosphine constituting the catalyst are purged at the same time. Consequently, process (1) is very undesirable from an economic viewpoint.
With respect to process (2), one such known process comprises introducing a large amount of a circulating reaction gas into a reactor which retains all of the high boiling point by-products, the catalyst, and the aldehyde produced, in order to carry out gas-stripping of the aldehyde produced and the high boiling point by-products under the hydroformylation condition (see Japanese Patent Application (OPI) No. 125103/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")). In this process, however, a large amount of circulating gas is required, particularly for the high boiling point by-products having a low vapor pressure. Further, the process requires a very difficult operation wherein the production amount and the removal amount of the high boiling point by-products must be strictly controlled based on the amount of the circulating gas.
According to the process (3), since extraction or adsorption always involves a problem of selectivity, the problem occurs that certain components can be separated while other components cannot be separated. Therefore, it is very difficult to effect industrial practice of this process.
Lastly, according to the process (4), there is a drawback that by-products having a boiling point higher than a certain value cannot be removed, although those having a boiling point up to a certain value can be removed. However, this drawback can be partially overcome by combining with the process (1).
Also, the organic high boiling point by-products form a homogeneous phase in the circulating catalyst liquid together with the Group VIII noble metal-triarylphosphine complex catalyst and the free triarylphosphine. Therefore, in order to effectively circulate the Group VIII noble metal-triarylphosphine complex and the free triarylphosphine to the reaction zone, it is important that not only are the high boiling point by-products selectively separated in the desired amount from the circulating catalyst liquid, but also that the catalytic activity of the circulating catalyst liquid should not be damaged in the separation step.